1. Field of the Invention
The present invention relates to a pneumatic tire for use on a truck and bus and more specifically to tread grooves of a tire for preventing stone retention.
2. Description of the Prior Art
When a tread portion of a large-sized radial tire for use on a truck or bus is provided with a reinforcing belt layer composed of tough steel cords, the rigidity of the tread portion is increased by the reinforcing layer, and movements of the tread are restrained causing the tread to easily retain stones which are very different to be removed. As a result, the groove bottom of the tread groove is damaged by the retained stones every time contact of the stones with the road surface is made during driving, and sometimes the damage reaches the reinforcing belt layer causing further damage to the steel cords of the belt layer.
The tire for use on a truck or bus is usually recapped by replacing the tread rubber when it has been worn, but it is impossible to recap a tire of which the belt layer is damaged, thus the life span of the tire is considerably shortened.
In order to prevent the tread groove of the tire from stone retention, a pneumatic tire having a groove in which a side wall of the tread groove is provided with not less than two steps has been proposed as disclosed in Japanese laid open Patent Publication No. 50-98002.
This known pneumatic tire having a groove with steps has indeed an advantage in the aspect of reducing the occurrence of stone retention at the earlier stage of the wear of tire, but a problem develops in that at later stage of the wear i.e., after the wear reaches the step portions, the groove is deformed to a shape similar to that of a conventional tread groove, causing stone retention, and the retained stone is hard to remove. Moreover, there exists another disadvantage in that when the wear of the tire advances to the step portions of the tread groove, the width of the groove is sharply narrowed to reduce the volume of the groove, thereby causing eases slipping.